1. Field of the Invention
The present invention relates to a motor driving device that converts AC power supplied from an AC power supply side to DC power and outputs the DC power to a DC link, and thereafter, further converts the DC power to AC power for motor driving and supplies the AC power to a motor. In particular, the present invention relates to a motor driving device having a PWM converter as a power converter for converting AC power on the AC power supply side to DC power.
2. Description of the Related Art
In a motor driving device that drives a motor within machine tools, forging machines, injection molding machines, industrial machinery, or various robots, AC power supplied from the AC power supply side is temporarily converted to DC power and thereafter is further converted to AC power, and the AC power thus obtained is used as driving power for a motor provided for each drive axis. The motor driving device includes a converter (rectifier) that rectifies AC power supplied from the AC power supply side to output DC power, and an inverter (reverse converter) that is connected with a DC link, which is the DC side of the rectifier, and that performs power conversion between DC power in the DC link and AC power as driving power for or regenerative power from the motor, wherein the speed and torque of the motor connected to the AC side of the inverter, or the position of a rotor are controlled. A capacitor is provided in the DC link, which connects the DC side of the converter and the DC side of the inverter. The capacitor has a function as a smoothing capacitor for suppressing the ripple components of the DC output of the converter, and a function as a power storage device that may store DC power. The same number of inverters are provided as the number of motors, in order to drive-control the motors by supplying the driving power to the individual motors respectively provided correspondingly to the plurality of drive axises. A single converter is generally provided for the plurality of inverters, in order to reduce the cost of the motor driving device and the space occupied.
In recent years, as a converter in a motor driving device, a PWM control type converter (hereinafter, simply referred to as a PWM converter) that enables regenerative power generated during motor deceleration to be returned to the AC power supply side has been widely used, in view of the demand for saving energy. The PWM converter has a power factor of approximately 1 and is advantageous in that a DC voltage in a DC link can be boosted up to a desired voltage equal to or more than an input voltage peak value of an AC power supply. The PWM converter is constituted of a bridge circuit having semiconductor switching elements and diodes connected in reverse parallel therewith, and performs power conversion between AC power on the AC power supply side and DC power in the DC link side with switching operations of the semiconductor switching elements therein being PWM-controlled. Regenerative power generated in a motor during deceleration of the motor is converted from AC power to DC power by an inverter, and the DC power is input to the PWM converter via the DC link and is further converted to AC power by the PWM converter, thereby being regenerated to the AC power supply side. At this time, the DC voltage in the DC link where a capacitor is provided varies corresponding to the amount of regenerative power generated in the motor and the amount of AC power regenerated to the AC power supply side via the inverter, the DC link and the PWM converter.
As described in, for example, Japanese Unexamined Patent Publication No. 2000-236679, there is known a motor driving device wherein a power storage device is provided in the DC link side of a PWM converter, the magnitude of input current (input power) to be fed from an AC power supply to the PWM converter by PWM control is limited, the peaks of power to be supplied from the AC power supply are suppressed by use of energy stored in the power storage device for accelerating a motor, and during deceleration of the motor, the peaks of power to be regenerated to the AC power supply side are suppressed, thereby achieving reduction in the capacity of the PWM converter.
In the motor driving device, when driving power necessary for motor acceleration is denoted by X[W] and a limit value of input power (corresponding to input current) to be fed from the AC power supply to the PWM converter by PWM control is denoted by Y[W], power is short by Z[W] (=X[W]−Y[W]) for motor driving but is supplied from the power storage device for the short power Z[W]. When a time period for accelerating the motor is denoted by T[s], energy E[J] to be supplied as the driving power for the motor from the power storage device is represented by expression (1).E=Z×T  (1)
When the driving power for the motor is supplied from the power storage device, a DC voltage (power storage device voltage) in the DC link drops. When a DC voltage in the DC link before the supply of the driving power for the motor from the power storage device is denoted by V1[V], a dropped DC voltage in the DC link after the supply of the driving power for the motor from the power storage device is denoted by V2[V], and a capacitance of a capacitor as the power storage device is denoted by C[F], relational expression (2) holds.
                    E        =                              1            2                    ×          C          ×                      (                                          V                1                2                            -                              V                2                2                                      )                                              (        2        )            
In addition, as described in, for example, Japanese Patent Publication No. 4917680, there is a motor driving device wherein a power storage device is charged with power from the AC power supply or regenerative power generated during motor deceleration, for the amount of DC voltage in the DC link (power storage device voltage) that has dropped when energy stored in the power storage device is used for motor acceleration, and the energy thus stored in the power storage device is used as driving power for next motor acceleration.
It is possible by PWM control to limit input current to be fed from the AC power supply side to a PWM converter only when the DC voltage in the DC link, which is the DC side of the PWM converter (i.e., power storage device voltage), is higher than an input voltage peak value on the AC power supply side of the PWM converter.
However, when the DC voltage in the DC link drops down to be equal to or lower than the input voltage peak value, current flows into diodes connected in reverse parallel with semiconductor switching elements and therefore the configuration is equivalent to that of a diode rectification-type converter. Consequently, even performing PWM control on switching operations of the semiconductor switching elements does not make it possible to limit the input current to be fed from the AC power supply side. Therefore, when motor acceleration is continued afterwards, all of the power necessary for the motor acceleration has to be fed from the AC power supply side and this makes it impossible to achieve the objective of suppressing the peaks of the power supplied from the AC power supply. In addition, since it may be impossible to limit the input current to be fed from the AC power supply side, there is a risk of breakage of the diodes in the PWM converter or the power storage device due to overcurrents.
FIGS. 10A to 10D are diagrams illustrating operation of a conventional motor driving device having a power storage device and a PWM converter that is provided in order to suppress the peaks of power to be supplied from the power supply during acceleration of the motor and the peaks of power to be regenerated to the power supply during deceleration of the motor, FIG. 10A illustrating a motor output, FIG. 10B illustrating a power supply current to be fed from the AC power supply to the PWM converter, FIG. 10C illustrating current flow in the power storage device, and FIG. 10D illustrating a DC link voltage.
Firstly, a case in which a DC voltage in a DC link has already been boosted up to an input voltage peak value of an AC power supply at time t0 is considered. In FIG. 10D, V1 represents a DC voltage in the DC link before driving power for the motor is supplied from the power storage device. V2 represents a dropped DC voltage in the DC link after the supply of the driving power for the motor from the power storage device and is set to a value larger than the input voltage peak value of the AC power supply side.
During a period between time t0 and time t1, the PWM converter converts AC power on the AC power supply side to DC power, while being PWM-controlled in a manner such that a power supply current (AC current) to be fed from the AC power supply does not exceed a preset input current limit value (FIG. 10B). The power storage device is charged with the DC power output from the PWM converter (FIG. 10C), and the DC voltage in the DC link (i.e. power storage device voltage) gradually increases (FIG. 10D).
After the DC voltage in the DC link reaches a prescribed voltage V1 at time t1, neither the power supply current (FIG. 10B) nor the power storage device current (FIG. 10C) flows until motor acceleration is started.
When motor acceleration is started at time t2 (FIG. 10A), the power supply current (input current) that is fed to the PWM converter from the AC power supply increases (FIG. 10B). No power storage device current flows (FIG. 10C) until the power supply current reaches the input current limit value.
When powering of the motor continues and the motor output continues increasing (FIG. 10A) even though the power supply current has reached the input current limit value at time t3 (FIG. 10B), the limited input power to be supplied from the AC power supply side is not enough for the driving power and, as a result, the shortage of the power is compensated by the power discharged from the power storage device. Accordingly, the power storage device current flows (FIG. 10C) and the DC voltage in the DC link (i.e. power storage device voltage) gradually decreases (FIG. 10D).
If the DC link voltage is greater than the input voltage peak value of the AC power supply when the motor output further continues increasing even though the DC link voltage falls below V2 at time t4, it is possible to limit by PWM control the magnitude of the input current to be fed from the AC power supply side to the PWM converter (FIG. 10B).
When the motor output further continues increasing even though the DC link voltage falls below the input voltage peak value at time t5, current flows through the diodes in the PWM converter and, as a result, it is impossible to limit the magnitude of the input current to be fed from the AC power supply side. Accordingly, breakage of the diodes in the PWM converter or the power storage device may occur due to overcurrents, or in order to avoid this, the motor driving device itself may be stopped by alarm.
In the above-described motor driving device having the PWM converter and the power storage device, it is important to perform control such that the DC voltage in the DC link that has dropped after the supply of the driving power for the motor from the power storage device (V2[V] in expression 2) does not fall below the input voltage peak value on the AC power supply side.
For example, such control is conventionally performed that the dropped DC voltage in the DC link (V2[V] in expression 2) does not fall below the input voltage peak value on the AC power supply side by designing a capacitance C[F] of a capacitor as the power storage device based on expression 1 and expression 2 as follows. In other words, the DC voltage V1[V] in the DC link before the supply of the driving power for the motor from the power storage device is set equal to or lower than withstand voltages of the diodes in the PWM converter and the power storage device, and the dropped DC voltage V2[V] in the DC link after the supply of the driving power for the motor from the power storage device is set to a value greater than the input voltage peak value on the AC power supply side and equal to or greater than a minimum voltage necessary for driving the motor. As for driving power X[W] necessary for motor acceleration and a time period T[s] during which the motor is accelerated are determined based on motor operation conditions, and a limit value Y[W] of the input power (corresponding to the input current) to be fed from the AC power supply to the PWM converter by PWM control is limited to power capable of being supplied by the AC power supply.
According to expression 2, making a difference V1[V]−V2[V] large between the DC voltages in the DC link before and after the supply of the driving power for the motor from the power storage device can reduce the capacitance C[F] of the capacitor as the power storage device. However, the DC voltage V1[V] in the DC link before the supply of the driving power for the motor from the power storage device needs to be set to a value smaller than the withstand voltages of the diodes in the PWM converter and the power storage device, and therefore has an upper limit. In addition, the DC voltage V2[V] in the DC link after the supply of the driving power for the motor from the power storage device needs to be set to a value larger than the input voltage peak value on the AC power supply side, and therefore depends on the AC power supply to be connected to the motor driving device. For example, in the case where the AC power supply is of high voltage, the DC voltage V2[V] in the DC link after the supply of the driving power for the motor from the power storage device may not be made low, and therefore the difference V1[V]−V2[V] between the DC voltages in the DC link before and after the supply of the driving power for the motor from the power storage device may not be made large. Accordingly, it is necessary to make the capacitance C[F] of the capacitor as the power storage device large in order to sufficiently ensure energy E[J] to be supplied as the driving power for the motor from the power storage device, resulting in a problem that the cost and installation area of the power storage device would be increased.
Further, by limiting the magnitude of the input current (input power) to be fed from the AC power supply to the PWM converter by PWM control and by using energy stored in the power storage device for the shortage of power for motor driving, the capacity of the PWM converter can be made small, and therefore it is possible to make a selection of a PWM converter having a capacity smaller than the motor output. However, when the motor output is larger than is assumed due to some cause or when the capacitance of the capacitor as the power storage device decreases due to aged deterioration, the situation may also be considered to occur that the DC voltage (power storage device voltage) in the DC capacitor is equal to or lower than the input voltage peak value of the AC power supply because of using the energy stored in the power storage device for motor acceleration and, as a result, a motor load would be directly applied to the PWM converter. In this case, if the selected PWM converter has a small capacity with respect to the motor output, it may be impossible to limit the input current (input power) to be fed from the AC power supply to the PWM converter, and therefore, there would arise a problem that breakage of the diodes in the PWM converter or the power storage device may occur due to overcurrents, or in order to avoid this, the motor driving device itself may be stopped by alarm.